RFR: 8224659: Parallel GC: Use WorkGang (1: PCRefProcTask)

[Kim Barrett]

On May 27, 2019, at 1:18 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On May 24, 2019, at 7:25 AM, Leo Korinth <leo.korinth at oracle.com> wrote:
>> […]
>> Enhancement:
>> https://bugs.openjdk.java.net/browse/JDK-8224659
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~lkorinth/workgang/0/_8224659-Parallel-GC-Use-WorkGang-1-PCRefProcTask/
>> http://cr.openjdk.java.net/~lkorinth/workgang/0/all/
>> 
>> […]
> 
> src/hotspot/share/gc/parallel/psParallelCompact.cpp   
> 2108   do {
> 2109     while (ParCompactionManager::steal_objarray(worker_id,  task)) {
> 2110       cm->follow_array((objArrayOop)task.obj(), task.index());
> 2111       cm->follow_marking_stacks();
> 2112     }
> 2113     while (ParCompactionManager::steal(worker_id, obj)) {
> 2114       cm->follow_contents(obj);
> 2115       cm->follow_marking_stacks();
> 2116     }
> 2117   } while (!terminator.offer_termination());
> 
> This might offer termination earlier than it should, e.g. even if some
> objarrays have become available while other objects were being
> processed.  Consider instead something like
> 
>  while (true) {
>    if (ParCompactionManager::steal_objarray(worker_id,  task)) {
>      cm->follow_array((objArrayOop)task.obj(), task.index());
>      cm->follow_marking_stacks();
>    } else if (ParCompactionManager::steal(worker_id, obj)) {
>      cm->follow_contents(obj);
>      cm->follow_marking_stacks();
>    } else if (terminator.offer_termination()) {
>      break;
>    }
>  }
> 
> or maybe make the second clause
> 
>    } else if (ParCompactionManager::steal(worker_id, obj)) {
>      do {
>        cm->follow_contents(obj);
>        cm->follow_marking_stacks();
>      } while (ParCompactionManager::steal(worker_id, obj)) {
>    }
> 
> to only check for arrays again when run out of normal objects (as done
> in the original proposed change).

Leo pointed out to me that the webrev code was cribbed from the pre-existing
StealMarkingTask::do_it().  So I’m okay with leaving the webrev code as-is, and
have asked Leo to create a followup RFE to look into changing this (and any
other similar places).